In the episode "I Borg", the Enterprise crew found a single Borg drone that was severed from the Borg Collective. They named him "Hugh" and taught him the value of individuality. Hugh was eventually returned to the Collective by the Borg, but his ideas of individuality spread throughout his ship and caused a lot of Borg drones to reject their collective mind and revert to their original individuality. This group is subsequently encountered in "Descent: Part 2" (1993), raising the question as to why not all Borg have been "freed" this way by the time Star Trek: First Contact takes place, since all Borg are connected by a subspace network. The reason may be a combination of factors. For one, it is known that that the Borg's collective consciousness will reject anything that would threaten their hive mind, as was demonstrated in Voyager episode "Unimatrix Zero: Part 1" (2000). Also, the idea of shared consciousness is particularly deeply rooted in the Collective and fiercely defended by older drones, especially when separated from the Collective (such as Seven of Nine displayed in Voyager episode "Survival Instinct" and in her later life). And even after successful separation, some freed individuals still long to a form of collective mind (Voyager episode "Unity"). Consequently, Hugh may have introduced his radical idea of individuality into the entire Collective, but it can be expected that many drones, and therefore the hive mind, would largely resist this idea. They would even take measures to eliminate this dangerous thought from their consciousness and destroy drones that have embraced the idea and continue to spread it. Therefore, it is more plausible that Hugh had to introduce the idea subtly, perhaps one drone at a time, to see if the idea would stick and, at the same time, remain undetected. Of course, the rest of the Borg would inevitably find out at one point that drones were breaking off from the Collective and would take measures against it, which explains why the Borg Collective is still largely intact.